Radiotracked birds

When radio-tagged migrating birds are followed in a small aeroplane, considerable detail on day-to-day behaviour can be obtained, in addition to measures of speed. In one of the earliest studies, 10 radio-marked Hylocichla thrushes (of three species) were followed on their nocturnal migrations over favourable terrain in North America (Cochran et al. 1967). Some birds migrated for 4-8 hours per night, over distances of about 180-540 km, while others flew for less than an hour each night (especially when interrupted by bad weather). In another study, seven individual Peregrines Falco peregrinus were followed from a small plane while they were migrating across the USA. In each 24-hour period, these birds typically spent 17 hours on a perch, 6 hours in migratory flight, and about 1 hour in hunting. In especially favourable conditions, migration increased to 9 hours per day, and when held up by weather, perching increased to 23 hours per day. On average, these falcons migrated on six days out of seven, generally from mid-morning to late afternoon (W. Cochran, records compiled by White et al. 2002).

In another study, 15 radio-marked adult Bald Eagles Haliaetus leucocepha-lus were tracked from a vehicle for an average distance of 2019 km in spring (Harmata 2002). These birds migrated individually (like those just mentioned), appeared not to feed during the journey, and did not migrate on days of overcast or high wind. All their flights occurred at some time within a 9-hour period each day (10.30-17.30 hours), covering an average of 180 km (range 33-435 km), at an average flight speed of 50 km per hour (range 20-144 km per hour). The birds flew at altitudes of 30-4572 m above ground, but mostly at 1500-3050 m, so for most of the time they were beyond the range of human vision.

Figure 8.5 Course of first-year migration in White Storks Ciconia ciconia from southwestern Germany (westerly route) and northwestern Germany (easterly route), showing the migratory divide. Dots show the average monthly location of ringed birds. The lines join the dots of successive months, but do not necessarily reflect the exact routes followed. First-year storks are on the move for much of the year, and most do not reach the breeding areas until it is too late to nest that year. From Bairlein (2001).

Figure 8.5 Course of first-year migration in White Storks Ciconia ciconia from southwestern Germany (westerly route) and northwestern Germany (easterly route), showing the migratory divide. Dots show the average monthly location of ringed birds. The lines join the dots of successive months, but do not necessarily reflect the exact routes followed. First-year storks are on the move for much of the year, and most do not reach the breeding areas until it is too late to nest that year. From Bairlein (2001).

Other data from radio-marked birds refer to individuals mostly tracked from satellites, in which records of locations came at longer intervals, and without the behavioural details. Such studies have involved only large species, complementing the data from ringing which refer mainly to small species. At face value, the two types of data provide only weak support of the theoretical prediction that smaller species migrate more rapidly than larger ones. However, both data sets exclude the initial fattening period (which can extend to several weeks in some large species). Also, most records refer to autumn, when many birds seem not to migrate at their maximum possible rate, but to linger in areas of plentiful food which they encounter en route. Even in spring, when birds are assumed to migrate as rapidly as possible to reach their breeding areas, the migration speeds of passerines are generally no faster than those of larger waterfowl and others (including raptors and cranes which travel by soaring flight). The data are more consistent with the view that spring speeds are largely influenced with the spread of warmth to higher latitudes (as reflected, for example, in the northward movement of particular isotherms, Chapter 14). On this view, species would move to progressively higher latitudes as conditions allowed, with no obvious differences between small and large species, whatever their theoretical capabilities.

Radio-tracking studies have also revealed great variation in migration speeds among adults from the same population making essentially the same journey, some individuals taking 2-3 times longer than others, or occasionally up to 12 times as long. Much of this variation was associated with the weather encountered en route, birds departing at different dates encountering different conditions, but some may also have been associated with the varying capabilities of the birds themselves. The inclusion of juveniles in the comparison would in some species have increased the variation in migration speed even more. Some studies have involved species that migrate by flapping flight, while others have involved species that migrate mainly by soaring-gliding flight. The two groups are discussed separately below.

Birds that migrate by flapping flight

The average speeds of four waterfowl species tracked over land in spring averaged 55-118 km per day (Table 8.2). Individual variation was sometimes great, as illustrated by Bewick's Swans Cygnus columbianus bewickii migrating between northern Russia and the Netherlands. In spring two birds tracked along part of the route gave mean migration speeds of only 28 and 39 km per day. In autumn, five birds followed over the entire 3200 km journey took 41-78 days, giving average speeds of 44-72 km per day. However, observational records of departure and arrival dates indicated that some flocks under favourable winds completed the whole 3200-km journey in about eight days (400 km per day). Hence, the individuals from this population varied between eight and 78 days in the time taken to complete their autumn journey, giving average speeds of 44-400 km per day, a nine-fold variation depending on the amount of stopover, which in turn depended largely on the weather. In addition, pairs with young took longer over the journey than adults alone, stopping more frequently and arriving later in the wintering areas: probably juveniles were less able than adults to cover such long distances non-stop (Beekman et al. 2002).

Table 8.2 Details from the tracking of radio-tagged waterfowl and other birds on their migrations

Species

Migration route

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